Recent advances in color copying and printing have put increasing importance on developing new methods to prevent forgery of security documents such as banknotes. While there have been many techniques developed, one area of increasing interest is in developing security features that cannot be readily reproduced, particularly by a color copier or printer.
One approach that has been taken is to formulate an ink for creating a printed image that is visually distinct from its reproduction. For example, U.S. Pat. Nos. 5,059,245, 5,569,535, and 4,434,010, the entireties of which are incorporated herein by reference, describe the use of stacked thin film platelets or flakes. Images produced with these pigments exhibit a change in color as the viewing angle changes. These pigments have been incorporated into security inks used, for example, in paper currency. These pigments have also been incorporated into plastics applications (see, for example, PCT Publication WO 00/24580, published May 4, 2000). Additional inks and security features are described in U.S. Pat. Nos. 4,705,356; 4,779,898; 5,278,590; 5,766,738; and 6,114,018, the entireties of which are incorporated herein by reference.
Direct write printing processes such as ink jet printing are highly desirable for various security applications because they provide the ability to print variable features and/or in formation, such as serial numbers. Inks containing platelets and flakes, however, are generally undesirable for certain printing processes, such as ink jet printing processes, because the flow characteristics of inks containing platelets and flakes are generally unsuitable for such deposition techniques. Accordingly, the need exists for inks suitable for direct write printing processes, such as ink jet printing processes, to form images that exhibit color shifting. The need also remains to provide additional processes, e.g., ink jet printing processes, for producing images, particularly for security documents, which cannot be easily reproduced, and which are visually distinct from their reproductions.
Additionally, the need exists for providing the ability to create features, e.g., security features, that display variable information, e.g., information that is individualized for a specific product unit, such as a serial number, which variable information cannot be easily or readily duplicated or copied. The need also exists for providing the ability to create reflective features displaying variable information and having high resolution at commercially acceptable rates.